
CARNOT’S THEORY OF THE MOTIVE POWER OF HEAT. 543 
“In our demonstrations we tacitly assume that after a body has experienced 
a certain number of transformations, if it be brought identically to its primitive 
physical state as to density, temperature, and molecular constitution, it must 
contain the same quantity of heat as that which it initially possessed; or, in 
other words, we suppose that the quantities of heat lost by the body under one set 
of operations are precisely compensated by those which are absorbed in the others. 
This fact has never been doubted ; it has at first been admitted without reflection, 
and afterwards verified, in many cases, by calorimetrical experiments. To deny 
it would be to overturn the whole theory of heat, in which it is the fundamental 
principle. It must be admitted, however, that the chief foundations on which the 
theory of heat rests, would require a most attentive examination. Several expe- 
rimental facts appear nearly inexplicable in the actual state of this theory.” 
7. Since the time when Carnot thus expressed himself, the necessity of a 
most careful examination of the entire experimental basis of the theory of heat 
has become more and more urgent. Especially all those assumptions depending 
on the idea that heat is a substance, invariable in quantity ; not convertible into any 
other element, and incapable of being generated by any physical agency; in fact 
the acknowledged principles of latent heat; would require to be tested by a most 
searching investigation before they ought to be admitted, as they usually have 
been, by almost every one who has been engaged on the subject, whether in com- 
bining the results of experimental research, or in general theoretical investigations. 
8. The extremely important discoveries recently made by Mr Jou of Man- 
chester, that heat is evolved in every part of a closed electric conductor, moving 
in the neighbourhood of a magnet,* and that heat is generated by the friction of 
fluids in motion, seem to overturn the opinion commonly held that heat cannot 
be generated, but only produced from a source, where it has previously existed 
either in a sensible or-in a latent condition. 
* The evolution of heat in a fixed conductor, through which a galvanic current is sent from any 
source whatever, has long been known to the scientific world; but it was pointed out by Mr Jou.e 
that we cannot infer from any previously-published experimental researches, the actual generation of heat 
when the current originates in electro-magnetic induction ; since the question occurs, is the heat which is 
evolved in one part of the closed conductor merely transferred from those parts which are subject to the 
inducing influence? Mr Jouve, after a most careful experimental investigation with reference to 
this question, finds that it must be answered in the negative-—(See a paper ‘* On the Calorific Effects 
of Magneto-Electricity, and on the Mechanical Value of Heat; by J. P. Journ, Esq.’ Read before 
the British Association at Cork in 18438, and subsequently communicated by the Author to the 
Philosophical Magazine, vol. xxiil., pp. 263, 347, 435.) 
Before we can finally conclude that heat is absolutely generated in such operations, it would be 
necessary to prove that the inducing magnet does not become lower in temperature, and thus com- 
pensate for the heat evolved in the conductor. I am not aware that any examination with reference 
to the truth of this conjecture has been instituted; but, in the case where the inducing body is a 
pure electro-magnet (without any iron), the experiments actually performed by Mr Jouve render 
the conclusion probable that the heat evolved in the wire of the electro-magnet is not affected by 
the inductive action, otherwise than through the reflected influence which increases the strength of 
its own current. 
